1. Field of the Invention
The present invention relates generally to apparatus for measuring the thickness of materials, and is particularly concerned with a collapsible restraint which can be attached to a flexible measuring tape for measuring the thickness of an ice cover over a lake or other body of water.
2. Description of the Prior Art
It is common in the art to measure the thickness of a material by measuring the depth of a hole provided in the material, and a number of different devices have been employed for this purpose. In U.S. Pat. No. 2,470,498, for example, a device is disclosed for measuring the thickness of the refractory brick layer which forms the roof of an open hearth furnace, such furnaces being subject to deterioration by erosion which results in an eventual reduction in the thickness of the roof. The device is provided in the form of an elongated vertical tube which encloses a slide rod having a handle at its upper end and a pivoting detent at its lower end. The detent can be pivoted by means of the handle between a retracted position in which the detent is confined within the walls of the outer tube and a deployed position in which the ends of the detent project through slots in the side walls of the tube. In use, the detent is maintained in the retracted position and the vertical tube is inserted into a bore provided in the furnace roof until the slots in the tube have moved past the bottom opening of the bore. The handle is then operated to move the detent to its deployed position, whereupon the device as a whole is raised to bring the detent into engagement with the bottom edges of the hole. A measuring rod slidably attached to the upper portion of the outer tube is then used to obtain a measurement of the hole depth, which provides an indication of the amount of erosion which has taken place on the inner surface of the furnace.
Although the foregoing type of device is useful for measuring the thickness of slowly eroding walls and the like, where the material thickness will not be expected to vary a great deal between successive measurements, it is not well suited for applications in which the material thickness may vary over a wide range. An ice layer covering a lake or other body of water, for example, may be less than an inch thick or several feet thick, depending on climatic conditions and other factors. In order to measure the thickness of such ice layers, the device of U.S. Pat. No. 2,470,498 would have to be made long enough to completely penetrate an ice layer of the maximum expected thickness, which would result in a device of unmanageably large size.
In measuring the thickness of ice layers over bodies of water, it has been the practice to employ a flexible measuring tape having a steel rod attached to one end thereof. The rod and tape are passed through a hole which has been drilled in the ice cover, with the rod maintained in a generally vertical position to clear the sides of the hole. With the aid of an additional length of string or wire attached to the rod, the rod is positioned horizontally to bridge across the bottom of the hole and to provide a restraint for the tape. The tape can then be drawn up taut in order to allow the thickness of the ice layer to be measured. After the measurement has been completed, the string or wire is manipulated to position the rod vertically once again, whereupon the rod, tape and the additional length of string or wire are pulled back through the hole and up to the surface.
The foregoing type of material thickness measuring apparatus is advantageous because it can be used to measure ice layers of widely varying thickness, the range of measurement being limited only by the length of the measuring tape. Of course, since the measuring tape is flexible and can be easily rolled up for storage, it can be made quite long without rendering the apparatus difficult to store or transport. Some difficulty is encountered, however, in the use of the additional length of string or wire to move the rod between the vertical and horizontal positions. Since the position of the rod is not usually apparent to the user when the rod is located at the bottom opening of the hole in the ice layer, it is often necessary to manipulate the string or wire several times in order to place the rod in the proper vertical orientation for removal from the hole. This is a rather cumbersome procedure which results in an undesirable increase in the amount of time needed to complete the measurement procedure.